1. Field of the Invention
The present invention relates to methods and devices to effectively manipulate light with light alone while keeping the device dimensions small. To this end, a great deal of effort has been spent on identifying materials with strong optical nonlinearities and developing micron-scale components that amplify the optical response where the light-matter interaction is weak. For integration purposes it is also desirable that photonic functions are based on phase modulation rather than absorption so that the carrier-signal loses no energy in the process. The conventional approach has been to exploit relatively weak optical nonlinearities of the established photonic materials to control the propagation of optical signals with high-intensity pumps.
The present invention also relates to methods and devices for all-optical storage media such as interferometric bacteriorhodopsin films or photon storage in slow-light devices.
The present invention also relates to methods and devices that use optical functionality in organic and biological materials. Superior performance can be expected from biomaterials such as the photochromic protein bacteriorhodopsin (bR). Their photoresponse, characterized by high quantum yields and fatigue resistance, has been optimized by evolution and explored in a variety of optical applications. It is equally important that optically functional biomolecules are often found in organized self-assembled structures such as membranes.
The present invention also relates to methods and devices for light manipulation such as photonic crystals, resonant couplers, waveguide couplers, etc.
2. Brief Description of the Related Art
All-optical switches and logic gates are needed in photonic circuits that control the flow of light at near-infrared (IR) frequencies. Recent advances in optical device-miniaturization exemplified by the emergence of photonic bandgap nanostructures suggest molecules as possible functional components. A wealth of optical functionality has been discovered in organic and biological molecules. Superior performance can be expected from biomaterials such as the photochromic protein bacteriorhodopsin (bR). Their photoresponse, characterized by high quantum yields and fatigue resistance, has been optimized by evolution and explored in a variety of optical applications. It is equally important that optically functional biomolecules are often found in organized self-assembled structures such as membranes that can be readily crafted onto surfaces of existing photonic components or formed de novo. The expected weak optical response of a single molecular layer, however, has prevented the use of molecular self-assemblies for the direct modulation (control) of light propagation in photonic microstructures.
In parallel with these developments, microcavities with Q-factors of up to ˜1010 have found applications in solid-state optical switches, low-threshold lasers, and ultra-sensitive optical biosensors. In biosensing, binding of only a few molecules can shift the resonant frequency of the microcavity. It would be intriguing to extend this approach and use its high sensitivity to monitor structural changes within molecules, such as charge transfer and isomerization. Fast and reversible photoinduced structural changes characteristic of photochromic materials are particularly interesting as they imply the use of molecular systems for all-optical switching. Photoinduced transformation of a photochromic molecule between two isomers alters the absorption spectra and other physicochemical properties such as refractive index, oxidation/reduction potential, molecular conformation and fluorescence. Because of this diversity of photo-controllable properties photochromic molecules are poised to play an important role as functional elements in components of integrated photonic devices. Such applications generally require high speed, thermal irreversibility and high-fatigue resistance. Substances such as diarylethenes and a transmembrane protein bacteriorhodopsin (BR) endure numerous photocycles (104 and 105, respectively) and therefore hold the necessary robustness for use in optical devices. Consequently, the photoinduced anisotropies of these materials have been exploited extensively for ultra-fast photonic switching in the visible band. Based on the absorption shift, induced birefringence and changes in the refractive index in the demonstrated techniques cannot be directly extended into the telecommunication band (1,310/1,550 nm) where the known photochromic compounds are virtually transparent. The BR membrane protein encloses a retinal chromophore that is covalently bound to Lys216 residue via a protonated Shiff base (SB). More specifically, seven surrounding trans-membrane α-helices fix the molecular axis of retinal at an angle of θ=67° from the membrane plane normal as indicated in FIG. 9(a). Upon illumination BR undergoes a complex photocycle involving isomerization of the chromophore and proton transfer across the lipid membrane. The photocycle can be simplified as a bistable molecular switch due to the significant lifetimes of the thermodynamically stable planar all-trans protonated SB found in the bR state of the protein, and the metastable 13-cis deprotonated SB found in the protein's M state. Retinal conformation can be controlled optically by illuminating the protein around the maximum absorption wavelengths of 568 nm and 412 nm, driving BR directly into the M and bR state, respectively:
In the wild-type BR the M state relaxes thermally into the bR state through several spectroscopically distinct intermediaries within 10 ms. The relaxation timescale can however be altered by genetic mutations and controlling the pH of the solution as exemplified by the significantly prolonged M-state lifetime from 500 ms at pH=5 to 12 seconds at pH=8in the D96N mutant.
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